1. Field of the Invention
The present invention relates to a suspension of a vehicle such as an automobile, and more particularly to a control of an active suspension in relation to a turning of the vehicle.
2. Description of the Prior Art
As a type of hydraulic active suspensions of vehicles such as automobiles there is known, as shown in, for example, Japanese Patent Laid-open Publication 61-81212, a hydraulic active suspension comprising an actuator provided to correspond to each vehicle wheel so as to change vehicle height at a corresponding position according to supply or exhaust of a working fluid to or from a working fluid chamber thereof, a working fluid supply and exhaust means for supplying or exhausting the working fluid to or from said working fluid chamber, a transverse acceleration detection means for detecting actual transverse acceleration of a vehicle body, a vehicle speed detection means for detecting vehicle speed, a means for obtaining steering angular velocity, and a control means for calculating a differential value of an estimated transverse acceleration of the vehicle from the vehicle speed and the steering angular velocity, so as to control said working fluid supply and exhaust means according to a control amount based upon the actual transverse acceleration and a control amount based upon the differential value of the estimated transverse acceleration.
In such an active suspension, since the working fluid supply and exhaust means is controlled according to the control amounts based upon both the actual transverse acceleration of the vehicle body and the differential value of the estimated transverse acceleration of the vehicle, the rolling of the vehicle body in a transitional state of a turning is more effectively controlled as compared with a conventional active suspension in which the working fluid supply and exhaust means is controlled according to a control amount based upon only the actual transverse acceleration of the vehicle body.
However, when a vehicle makes a turn while it is running on a road having a very low surface friction coefficient (called hereinunder "low friction road"), although the differential value of the estimated transverse acceleration changes to a relatively great extent according to changes of the steering angle by a steering operation, the actual transverse acceleration changes little, and therefore, the control amount for the rolling of the vehicle body becomes excessive with a control amount corresponding to the control amount based upon the differential value of the estimated transverse acceleration, thereby causing an inverse rolling of the vehicle body or an unnatural roll change of the vehicle body due to a discrepancy between changes of the control amount based upon the differential value of the estimated transverse acceleration and changes of the actual transverse acceleration.